US 1768948 A
Description (OCR text may contain errors)
July 1, 1930.
Filed Dec. 3, 1921 Attorneys Patented July 1, 1930 UNITED STATES FRANK o. BAUM, or SAN FRANCISCO, CALIFORNIA, ASSIGNOR 'ro wEs'rINGIIoUsE ELECTRIC AND MANUFACTURING COMPANY, A CORPORATION or PENNSYLVANIA HIGH-VOLTAGE INSULATOR Application filed December 3, 1921. Serial No. 519,592.
This invention relates to the insulation of conductors for high voltage power transmis- I sion lines, and particularly under outdoorv weather conditions.
In insulating devices for such lines, a dry surface provides a greater resistance than a wet surface, and therefore has a. greater insulating effect. If a surface between two points differing greatly in potential is divided into wet and dry sections, the electrical stress will be largely concentrated upon the dry sections with the result that the air along the dry sections breaks down successively until an arc occurs, which because of its heat is frequently disastrous to the insulators that are usually of porcelain. The ideal insulator is therefore one with a uniformly wet surface or one with a maximum dry leakage surface.
lVith insulator units for insulating high voltage conductors, it has been found in practice that very high electrical stresses are set up adjacent the metallic pins or connectors upon the under faces of the insulator bodies and these stresses frequently form coronas which extend radially outward across the insulator surface. That is, the corona adjacent the metallic connector breaks down the air and this breakdown of the air lowers the resistance and transfers the electrical stress further out and causes a further breaking down of the air with the result that the corona creeps outwardly along the under surface. Corona, because of the fact that in the presence of moisture it forms acids, is a source of danger when it can spread in this manner.
The small -circular corrugations, commonly placed upon the under surfaces of insulators,
fail to effectively preventthe spread of the corona, it having been found in practice that the corona willspread along the surface to the nearest corrugation and then jump from corrugation to corrugation across the surface.
an insulating devicefoi high volta e conductors with which the outward sprea of corona is limited or prevented, which has suflicient dry surface under ordinary outdoor weather conditions to prevent a material brea ing down of the insulating effect, and which pre- ,vents the concentration of the potential across small dry surfaces when subjected to driving rain. A further object is to provide an insulating device in which a maximum possible cleansing action of rains upon the surfaces of a porcelain skirtintegral with the porcelain body of the unit at a point closely adjacent the connector, and extending down- "wardly a predetermined distance and outwardly until its inner surface has a length at leastapproximately equal to the distance from said point to a metallic enlargement on the connector below the insulator body, which in the cap and pin type, may be the cap of the I I also contemplate connext lower unit.
structing these units of different diameters,
so that when arranged'in the string with the larger units interspersedbetween the smaller units, the larger units will overhang andvat least partially protect the smaller units against light rains and against drip from the units above it. I may also provide insulator units with a plurality of transversely disposed rain shedding flanges having their upper andunder surfaces inclined upwardly to ward the axis and the inner edges of the sur.,
faces which face one another connected by ea'sy curves without sudden or sharp changes in direction, so that rain striking an upper surface will be deflected against the under surface of the flange immediately above it and at least partially wash the same. The shield \may be considered as a flange and its LPPBI surface is connected at its inner edge wlth the under surface of the flange immediately above it by an easy curve to facilitate cleansing of the said under surface under driving rains. The flanges are also preferably made of diiferent diameters so that a portion of them will overhang the others and protect them against ordinary rains and against drip from those above it.
In the accompanying drawings- Figure 1 is a sectional elevation of a portion of an insulator string with the insulator units constructed in accordance with my invention; a
Figure 2 is a sectional elevation of a portion of a string with a slightly modified insulator unit;
Figure 3 is a sectionalelevation of an insulator bushing constructed in accordance with my invention; and
Figure & is a sectional elevation of a slightly modified type of insulator bushing.
In a usual insulator unit ofthe cap and pin type, the body of insulating material extends like a flange from a central or boss portion. In some forms the under surface inclines directly from the periphery to the pin connector, and in others it may have depending circular ribs. WVith units of this type it has been found in practice that high electrical stresses are set up at points on the under surface immediately surrounding the connecting pin. These stresses form corona on the under surface which tend to extend radially outward to the periphery of the unit.- That is, the high electrical stress surrounding the pin tends to break down the air immediately adjacent thereto and the breakdown lowers the resistance so that the high electrical stress is extended towards the periphery of the unit. A further section of the air is then broken down so that the air will be broken down successively and permit of the extension of the corona to the periphery of the unit. The ribs do not limit the movement of the corona, and it has been found by test that the corona tends to extend along the under surface of the porcelain body until it reaches the tip of the rib first encountered, after which it jumps successively to the tips of the other ribs and then to the periphery of the insulator unit. In the presence of moisture the formation of acids by the corona is a source of danger when the corona can spread over the surface in this manner.
In Figures 1 and 2 I have shown an insulating device in which the formation of corona iseither prevented or its spread across the under surface of the unit limited. The units, which by way of illustration are shown as of the cap and pin type, comprise a body 1 of insulating material, such as of porcelain, having a central boss 2 over which is secured a metallic cap 3 for the suspension of this unit from the next higher unit, or from the support. A. connector pin l is secured in a recess 5 centrally beneath the cap 3 in a suitable manner such as by cement, and serves to support therefrom the next lower unit or the conductor through its pivoted connection with the ears 6 of the cap of said lower unit or with the conductors. The insulating body is provided with a transversely disposed flange 7, the surfaces of which both extend outwardly and downwardly, so that the upper surface thereof will shed the rain. The body 1 is also provided with a second flange in the nature of a corona shield S which extends downwardly from the under surface thereof at a point adjacent the connector pin. This shield may conveniently comprise an integral skirt which extends obliquely away from the connector pin at its point of connection with the porcelain body, with its inner surface of a length at least approximately equal to the distance of the metallic cap or enlargement in the connector from said point. The shield constructed in this manner will have a surface-creepage rupture strength between the points 6 and f at least equal to the rupture strength of the dielectric at the arcing zone, indicated as disposed along the line 9, between the insulator and the lower metallic parts under wet conditions, and, at the same time, permits a string of similar insulator units to be assembled in a string of substantially minimum length. Since the potential from the connector to the point f is limited in this manner I may give the skirt an inclination to the pin such that it will prevent corona, but if any corona does form, it will build down successively along the under side of the skirt only to an extent in which its potential difference from the point 0 equals that across the air at the distance 9' between the metallic counector andthe shirt. Practical tests have indicated that such a skirt effectively limits the action of the corona and prevents a breakdown of the insulator, especially under wet conditions.
The insulator-units may be made of different diameters so that units of greater diameter than the others may be interspersed along the string whereby the smaller units may be shielded from ordinary rains and from drippage from the units above them. At the same time the driving rains may engage the upper surfaces of the smaller units, as well as the larger units and cleanse them from accumulations of dust, etc. Each under surface exceptthe lowermost one is cleansed by the'driving rain deflected from the upper surface immediately below it. This deflection ofdriving rain is facilitated by connecting the inner edges of the under surface of the rain shedding flange and the upper surface of the shield flange by an easy or free curve 8, i. e., a curve without sharp or sudden changes in its rate of change of curvature, so that a driving rain engaging with the upper or outer surface of the shield will not only spatter upon the upper surface but will also be deflected upwardly along the connecting curve and then outwardly across the under surface of the flange.
v edge with the inner edge of the upper surface of the flange immediately below itrby a free or easy curve 12, for deflecting driving rains to cleanse the under surfaces of the flanges, in a manner similar to that effected by the curve 8 (Figure 1). The. under surface of the skirt or corona shieldll extends obliquely and upwardly toward the connectorpin 13. f
In Figure 3 I have illustrated the application of the invention to bushings for use on transformers, switches, and the like, for the insulation of high voltage conductors. The
- body- 14 of each unit is of suitable insulating k the corona shield may be omitted. The upper and lower surfaces of the flanges which face one another are connected by easy curves 12 similar to the curves 8 and 12 of the structures shown in Figures 2 and 3. Each unit is provided with an axial boss 16 at one end and a recess 17 at-the other end adapted to receive the boss 16 of another unit disposed in end to end relation therewith. These units are secured together end to end in a suitable manner, such as by cement 18 and the ali ned central longitudinal apertures 19 are l1ned with insulating sleeves of porcelain or other suitable material 20.
In Figure 4 a slightly modified type of bushing is illustrated in which the peripheral flanges 21 and 22 are alternately of greater and less diameter so that each flange of larger diameter shields the smaller flange immediately below it from ordinary rains. The under surface of each of these flanges is connected with the upper surface of the flange immediately beneath it by a free curve similar to the curves 8 and 12 of Figures 1 and 2 and for the same purpose..
In the insulating units of the types shown, the flanges which are overhung by those above them will be protected against ordinary rains so that a very large dry leakage surfacewill be available on each unit to provide the necessary resistance. Driving rains will engage the lower or outer portions of the flanges of smaller diameter and because of the free curves connecting the uppersurfaces of these flanges with the under surfaces of the flanges immediately above them, the driving rain engaging with the upper surfaces of the flanges will be deflected and directed along the free curves to a maximum extent and thereby cause a cleaning of the maximum possible area of the surface between two flanges. .The upward and inward inclination of the under surfaces of the flanges prevents the passage of the rain inwardly along the same by capillary action, so that the under surface of each flange will remain free from running moisture, except during driving rains in which case approximately the entire leakage surface will be wet. The leakage surface provided therefore closely approaches the ideal condition'of a uniformly moist surface or a surface having a maximum possible dry area.
It will be obvious that various changes in the details and arrangements herein described and illustrated may be made by those skilled in the art within the principle and scope of the invention.
1. An insulator comprising an insulating member having a plurality of laterally projecting portions having downwardlyandoutwardly-sloping upper and under surfaces, the under surface of one of said portions and the upper surface of the next lower thereof being spaced and formed relative to each other, and joined by a surface of rounded contour in accordance with the force of driving rain to cause the rain to travel in a stream therealong from said upper surface to said under surface.
2. An insulator comprising an insulating member having a plurality of superposed laterally-and-outwardly projecting annular flanges having downwardly-and-outwardlysloping upper and under surfaces, the under surface of one of said flanges and the upper surface of the next lower flange being spaced and formed relative to each other and joined by a surface of rounded contour in accordance with the force of driving rain to cause the rain to travel in a stream therealong from said upper surface to said under surface, the arrangement of surfacesitending to substantially uniformly wetthe entire outer surface of the insulator during a driving rain.
3. A high-voltage insulator comprising upper and lower terminal members, and an insulating member having a plurality of outer flanges having downwardly-and-outwardly-slopin upper and. under surfaces, the under sur ace of one of said flanges and the upper surface of the next lower thereof being spaced and' formed relative to each other andjoined by a surface of rounded contour in accordance with the force of driving rain to cause the rain to travel in a stream therealong from said upper surface to said under surface and the lowermost of I air across the shortest arcing zone between the lower terminal and the insulating member under wet conditions.
4. A high-voltage insulator comprising upper and lower terminal members, and an insulating member having an upper rainshedding flange and a lower corona-shielding flange constituting homogeneous parts thereof. the under surface of said rain-shedding flange and the upper surface of said coronashielding flange being spaced and formed relative to each other and joined by a surface of rounded contour in accordance with the force of driving rain to cause the rain to travel in a stream therealong from said upper surface to said under surface and the under surface of said corona-shielding flange hav ing a crcepage rupture strength therealong at least equal to the rupture strength ofthe air across the shortest arcing zone between the lower terminal and said corona-shielding flange under Wet conditions.
In witness whereof, I hereunto subscribe my signature.
FRANK G. BAUM.